1. Field of the Invention
This invention relates to silver halide photographic materials and, more particularly, to silver halide photographic materials having improved antistatic property without adversely influencing the photographic properties thereof.
2. Description of the Prior Art
A photographic material is composed of a support and silver halide photographic emulsion layers each having electric insulating property and hence it frequently occurs that electrostatic charges are accumulated when photographic materials are brought into contact with or are separated from the surfaces of the same photographic material or foreign materials during the production steps or the use of the photographic materials. The accumulated electrostatic charge causes various undesirable problems. For example, in undeveloped photographic films, the photosensitive silver halide emulsion layers of the photographic materials are exposed to the discharge light (spark) of the accumulated electrostatic charge when discharging of the electrostatic charge occurs and spot-like, branch-like or feather-like marks appear when the photographic films are developed. These marks are called "static marks" and the commercial value of photographic films is greatly reduced or is wholly lost as the case may be with the occurrence of the static marks. If such marks appear in medical or industrial X-ray films, etc., it may cause a misdiagnosis as will be easily recognized. This is a very troublesome problem since the occurrence of static marks can only be discovered after the photographic materials have been developed. Also, static charge which accumulates on film supports during production causes dust to attach to the surface of the film support, which induces secondary problems such as uneven coating, etc.
Such electrostatic charges frequently accumulate during the production or use of photographic materials as described above. For example, in the production of photographic films, electrostatic friction generates charges upon contact of the photographic films with rollers or upon the separation of support surfaces from the surfaces of silver halide emulsion layers during winding or rewinding of the photographic films. Also, in finished photographic films, electrostatic charges are caused by the separation of base surfaces from the surfaces of silver halide emulsion layers when the photographic films are wound up at a high humidity condition. In X-ray films electrostatic charges are caused by the contact or separation of X-ray films and mechanical parts or fluorescent screens in automatic X-ray cameras. Still further, electrostatic charges are also generated upon contact of photographic films with packing materials. The occurrence of static marks in photographic materials induced by the accumulation of such electrostatic charges becomes more severe when the sensitivity of the photographic materials is increased and the processing speed for photographic materials is increased.
The frictional charging is considered due to the electronic or ionic interaction of materials in contact but it is difficult at present to sufficiently anticipate based on structural chemistry what type of material will charge positively or negatively. However, it is clear that the occurrence of such charging can be prevented by reducing the charge potential or by increasing the electric conductivity of the surfaces of the materials such that electrostatic charges are released in a very short period of time prior to the occurrence of partial discharging of the accumulated charge. Therefore, processes for increasing the conductivity of the supports and various coating surfaces of photographic materials have been proposed and the utilization of various hygroscopic materials and water-soluble inorganic salts as well as certain kinds of surface active agents, polymers, etc., has been attempted. For example, there are known for the purpose the polymers described in U.S. Pat. Nos. 2,882,157, 2,972,535, 3,062,785, 3,262,807, 3,514,291 and 3,615,531; the surface active agents described in British Pat. No. 861,134 and U.S. Pat. Nos. 2,982,651, 3,428,456, 3,457,076, 3,454,625, 3,552,972 and 3,655,387; and zinc oxide, the semiconductors and colloidal silica, etc., as described in U.S. Pat. Nos. 3,062,700, 3,245,833 and 3,525,621.
As a process of directly imparting antistatic property to the supports for photographic films, there are known processes wherein the above-described materials are directly incorporated in the polymers used as supports for photographic films or are coated on the surfaces of the supports. In the latter case, the antistatic agents are coated on the supports for photographic materials as backing layers solely or as a combination with a polymer such as gelatin, polyvinyl alcohol, cellulose acetate, etc. Also, as an antistatic process for photographic materials, there is a process wherein the antistatic agent is incorporated in the photographic silver halide emulsion layers or the surface protective layers of the photographic materials or a solution of the antistatic agent is coated on the surfaces of these layers. However, the many antistatic materials as described above each show specificity according to the kind of film support used and the different photographic compositions and it sometimes occurs that they may show good results for one specific film support, photographic silver halide emulsion, and other photographic constituting elements but are utterly useless for the static prevention of other different film supports and photographic constituting elements.
On the other hand, it frequently happens that although some antistatic agents may have a very excellent antistatic effect, they cannot be used for photographic materials since they exert bad influences on the photographic properties of photographic silver halide emulsions, such as sensitivity, formation of fog, granularity, sharpness, etc., as well as they form scums in fix solutions. For example, it is known that polyethylene oxide series compounds, onium salts, etc., have antistatic effect but they frequently exert bad influences on photographic properties such as the increase of fog, desensitization, the reduction in granularity, etc.
In particular, in photographic materials having silver halide emulsion layers on the both sides of the supports, such as direct X-ray films, it is difficult to establish a technique of effectively imparting antistatic property to the photographic materials without reducing the photographic properties of the photographic materials.
Also, some of antistatic agents which may show a sufficient effect for the prevention of static marks immediately after coating show a reduction in antistatic faculty during the storage of, for example, medical direct X-ray films while inserting a paper between the films.
Furthermore, when the proportion of antistatic agents existing on the surfaces of photographic materials becomes relatively large, the antistatic agents are transferred to delivery rollers, cameras, screens, etc., which are brought into contact with the surfaces of the photographic materials, whereby various problems occur.
For example, when the antistatic agents are transferred to delivery rollers, they cause roller stain and cause sometimes attaching of the antistatic agents to the photographic films which passed through the rollers later.
Still further, in X-ray films the transfer of the antistatic agents causes important problems. If the antistatic agents are transferred to fluorescent intensifying screens, the screens are denatured and stains and unevenness are formed in the X-ray films after exposure, which reduces the quality of the photographic films as well as causes an erroneous diagnosis.